Studies on separation of various antigens of M. leprae gave a number of antigen positive fractions. One fraction appeared to possess a single antigen, and the others contained more than one antigen. Hypersensitivity and passive cutaneous anaphylaxis studies in guinea pigs sensitized with cell extracts from M. leprae and other mycobacteria indicated that the fraction containing single antigen was more specific. Studies on antibody forming cells (AFC) in mice infected with M. leprae indicated that the immunocytes are activated within a very short time after infection and peak about tenth day, increasing in numbers after secondary challenge. The high secondary response showed to be enhanced IgM rather than true IgG one. These investigations have indicated a need to obtain further information on the biologic activity of all the fractions and to obtain information on the status of the infected mouse in regard to the host-parasite association during progressive infection. The aims of the proposed study are as follows: 1. To extend studies on the various antigen active fractions with a view to confirming their specificity and cross reactivity as related to skin testing abilities. Efforts would be made to evaluate the antigens in respects of their ability to induce hypersensitivity as well as their immunogenic potential. 2. To compare the immune response exhibited by the mouse host infected with M. leprae with the immune response in mice infected with other slow growing mycobacteria. 3. To determine the possible status of the mouse host in the spectrum of human leprosy and to make efforts to assess the reasons, if any, as to why the mouse model behaves so by various immunologic tests.